1. Technical Field
The present disclosure relates to an improved micromechanical detection structure for a MEMS (Micro-Electro-Mechanical Systems) acoustic transducer, in particular a microphone of a capacitive type, and to a corresponding manufacturing process.
2. Description of the Related Art
As is known, a MEMS acoustic transducer of a capacitive type generally comprises a mobile electrode, provided as a diaphragm or membrane, set facing a substantially fixed electrode so as to form the plates of a detection capacitor.
The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a substrate, whereas a central portion is free to move or bend in response to acoustic-pressure waves impinging upon one of its surfaces. The mobile electrode and the fixed electrode provide a detection capacitor, and bending of the membrane that constitutes the mobile electrode causes a variation of capacitance of this detection capacitor. During operation, the variation of capacitance is converted, by suitable processing electronics, into an electrical signal, which is supplied as an output signal of the MEMS acoustic transducer.
A MEMS acoustic transducer of a known type is, for example, described in detail in US patent application No. 2010/0158279 A1 (which is incorporated by reference herein), filed in the name of the present Applicant.
FIG. 1 shows by way of example, and in a simplified manner, a portion of the micromechanical detection structure of the acoustic transducer, designated as a whole by 1.
The micromechanical structure 1 comprises a substrate 2 made of semiconductor material, and a mobile membrane (or diaphragm) 3. The membrane 3 is made of conductive material and faces a fixed electrode or rigid plate 4, generally known as “back plate”, which is rigid, at least when compared to the membrane 2, which is instead flexible and undergoes deformation as a function of incident acoustic-pressure waves.
Membrane 3 is anchored to substrate 2 by means of membrane anchorages 5, formed by protuberances of the same membrane 3, which extend from peripheral regions thereof towards the substrate 2.
For instance, membrane 3 has in a top plan view, i.e., in a horizontal plane of main extension, a substantially square shape, and the membrane anchorages 5, which are four in number, are set at the vertices of the square.
The membrane anchorages 5 perform the function of suspending the membrane 3 above the substrate 2, at a certain distance therefrom. The value of this distance is a function of a compromise between the linearity of response at low frequencies and the noise of the acoustic transducer.
In order to enable relief of the residual stresses (tensile and/or compressive stresses) on the membrane 3, for example deriving from the manufacturing process, through openings (not illustrated herein) may be formed through the membrane 3, in particular in the proximity of each membrane anchorage 5, in order to “equalize” the static pressure present on the surfaces of the same membrane 3.
The rigid plate 4 is formed by a first plate layer 4a, made of conductive material and facing the membrane 3, and by a second plate layer 4b, made of insulating material.
The first plate layer 4a forms, together with the membrane 3, the detection capacitor of the micromechanical structure 1.
In particular, the second plate layer 4b is set on the first plate layer 4a, except for portions in which it extends through the first plate layer 4a so as to form bumps 6 of the rigid plate 4, which extend as far as the underlying membrane 3 and have the function of preventing adhesion of the membrane 3 to the rigid plate 4, as well as of limiting the oscillations of the same membrane 3.
For instance, the thickness of the membrane 3 is in the range 0.3-1.5 μm, for example equal to 0.7 μm; the thickness of the first plate layer 4a is in the range 0.5-2 μm, for example equal to 0.9 μm; and the thickness of the second plate layer 4b is in the range 0.7-2 μm, for example equal to 1.2 μm.
The rigid plate 4 moreover has a plurality of holes 7, which extend through the first and second plate layers 4a, 4b, have, for example, a circular cross section, and perform the function of allowing, during the manufacturing steps, removal of underlying sacrificial layers. Holes 7 are, for example, set so as to form a lattice, in a horizontal plane, parallel to the substrate. Furthermore, in use, the holes 7 enable free circulation of air between the rigid plate 4 and the membrane 3, making the rigid plate 4 acoustically transparent. Holes 7 hence act as acoustic access ports in order to enable the acoustic-pressure waves to reach and deform the membrane 3.
The rigid plate 4 is anchored to the substrate 2 by means of plate anchorages 8, which are connected to peripheral regions of the same rigid plate 4.
In particular, plate anchorages 8 are formed by vertical pillars (i.e., extending in a direction orthogonal to the horizontal plane and to the substrate), which are made of the same conductive material as the first plate layer 4a and hence form a single piece with the rigid plate 4. In other words, the first plate layer 4a has prolongations that extend as far as the substrate 2 to define the anchorages of the rigid plate 4.
The membrane 3 is moreover suspended and directly faces a first cavity 9a, formed through the substrate 2, by means of a through trench etched starting from a back surface 2b of the substrate 2, which is opposite to a front surface 2a thereof, on which the membrane anchorages 5 rest (the first cavity 9a hence defines a through hole that extends between the front surface 2a and the back surface 2b of the substrate 2). In particular, the front surface 2a lies in the horizontal plane.
The first cavity 9a is also known as “back chamber”, in the case where the acoustic-pressure waves impinge first upon the rigid plate 4 and then upon the membrane 3. In this case, the front chamber is formed by a second cavity 9b, which is delimited at the top and at the bottom, respectively, by the first plate layer 4a and by the membrane 3.
Alternatively, it is in any case possible for the pressure waves to reach the membrane 3 through the first cavity 9a, which in this case performs the function of acoustic access port, and, hence, of front chamber.
In greater detail, the membrane 3 has a first surface 3a and a second surface 3b, which are opposite to one another and face, respectively, the first cavity 9a and the second cavity 9b, to be in fluid communication, respectively, with the back chamber and with the front chamber of the acoustic transducer.
Furthermore, the first cavity 9a is formed by two cavity portions 9a′, 9a′: a first cavity portion 9a′ is set at the front surface 2a of the substrate 2 and has a first extension in the horizontal plane; the second cavity portion 9a′ is set at the back surface 2b of the substrate 2 and has a second extension in the horizontal plane, greater than the first extension.
In a known way, the sensitivity of the acoustic transducer depends on the mechanical characteristics of the membrane 3, as well as on the assembly of the membrane 3 and of the rigid plate 4.
Furthermore, the performance of the acoustic transducer depends on the volume of the back chamber and the volume of the front chamber. In particular, the volume of the front chamber determines the upper resonance frequency of the acoustic transducer, and hence its performance at high frequencies. In general, indeed, the smaller the volume of the front chamber, the higher the upper cut-off frequency of the acoustic transducer. Furthermore, a large volume of the back chamber enables improvement of the frequency response and the sensitivity of the same acoustic transducer.
The present Applicant has found that the micromechanical structure 1 of a known type described previously is subject to some drawbacks, related in particular to manufacturing of the rigid plate 4 and to its anchorage to the substrate 2.
In particular, it is known that in a capacitive microphone the rigid plate (or reference plate) should be as planar as possible (given that it forms the reference plate of the detection capacitor), and moreover as rigid as possible (in order to prevent movements correlated to the acoustic-pressure waves or other undesired movements).
However, in the micromechanical structure 1 described previously, the rigid plate 4 may undergo a bending stress, as shown by the curved arrow represented with a solid line in FIG. 1, on account of the conformation of the plate anchorages 8, in particular of the aspect ratio, and of the suspended arrangement above the membrane 3, as well as on account of the residual stresses in the constituent materials, which determine a force acting in the direction indicated by the dashed arrows.
The factors affecting the aforesaid residual stresses are multiple and are due, for example, to the properties of the materials used, to the techniques of deposition of the same materials, to the conditions (temperature, pressure, etc.) at which deposition is carried out, and to possible subsequent thermal treatments.
In other words, a sort of spring, or elastic element, is formed at the plate anchorages 8, i.e., at the vertical pillar portions of the rigid plate 4 towards the substrate 2.
On account of its mechanical deformation, the rigid plate 4 may have a lower stiffness and moreover may not be perfectly planar or horizontal, thus affecting, even significantly, the performance of the acoustic transducer, for example reducing its sensitivity.
Furthermore, from the standpoint of the manufacturing process of the micromechanical structure 1, it is evident that formation of the plate anchorages 8, given their conformation, requires a large thickness of a resist layer, during the step of lithographic definition (the so-called “patterning”) of the last layers, or levels, of material.
In particular, this makes control of the manufacturing process problematical and moreover generates markedly vertical geometries, which may be particularly critical for chemical etchings (in particular, dry etches), considerably increasing the time required for execution of the same etchings.